Integrated: 8301489: C1: ShortLoopOptimizer might lift instructions before their inputs

[Daniel Skantz]

On Thu, 15 Jun 2023 11:13:02 GMT, Daniel Skantz <duke at openjdk.org> wrote:

> ShortLoopOptimizer might lift instructions before their inputs on some graph shapes. We propose adding a check that the insertion point for an instruction that is a candidate for hoisting should not be higher up the dominator tree than any inputs to the instruction.
> 
> Testing: tier1-tier3.
> 
> Additional testing: observed that `(cur_invariant && !v.is_valid())` never occurs on tier1-tier3 before the added test case.
> Also verified that the depth check is equivalent to `(*vp->block() == _insert->block()) || dominates(*vp, _insert)` on all of tier1-tier3.
> 
> Failure case: in the attached image the `arraylength` instruction from B10 is lifted to B0, as the dominator of B10 is calculated as B0. This is based on the logic in [`ComputeLinearScanOrder::compute_dominator_impl`](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/c1/c1_IR.cpp#L801).  But the array input is in Block 3. This is later spotted in `c1_LIRAssembler.cpp` with `Error: ShouldNotReachHere()`. We can reproduce the error on other instructions too -- the reader may refer to the test case provided.
> 
> ![image](https://github.com/openjdk/jdk/assets/111436254/9130516c-073d-45d1-a38a-af776e5e6672)

This pull request has now been integrated.

Changeset: 73d7aa1d
Author:    Daniel Skantz <daniel.skantz at oracle.com>
Committer: Tobias Hartmann <thartmann at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/73d7aa1d2cb037fed69263a1990258866333664d
Stats:     144 lines in 2 files changed: 143 ins; 0 del; 1 mod

8301489: C1: ShortLoopOptimizer might lift instructions before their inputs

Reviewed-by: thartmann, rcastanedalo

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PR: https://git.openjdk.org/jdk/pull/14492